1. Field of the Invention
The instant disclosure relates to an analog front end; in particular, to a pipelined analog-to-digital converter incorporating variable input gain and a pixel read out analog front end having the same.
2. Description of Related Art
An optical computer mouse uses a light source, typically a laser, and a light detector, such as a pixel array of photodiodes, to detect movement relative to a surface. A pixel read out front end is utilized to convert the analog signals of the pixel array to digital signals. Please refer to FIG. 1 showing a block diagram of a conventional pixel read out analog front end. The conventional pixel read out analog front end comprises a pixel array 1, transfer amplifier (TA) 2 and an analog-to-digital converter (ADC) 3. A source follower between the pixel array 1 and the transfer amplifier 2 is neglected in FIG. 1. The pixel array 1 is normally arranged in a square matrix with equal number of rows and columns (for example, 15 rows by 15 columns for a total of 15×15=225 pixels). After being exposed to external light sources reflected off an image surface 4, each of the pixels (a type of photo detector) converts the light signal to a voltage signal stored on a storage capacitor. The stored voltage signal in every pixel is read out serially through the transfer amplifier 2 and ADC (typically a pipelined ADC for high speed data conversion) 3 starting from the first row to the last row and in each row from the first column to the last column. A frame consists of the complete read out of all the pixels in the pixel array. The function of the programmable transfer amplifier 2 is to amplify the pixel voltage signal by a gain ranging typically between 1 and 3 before the analog-to-digital conversion for further digital signal processing. Since all the pixels in the pixel array share a common transfer amplifier (TA) and analog-to-digital converter (ADC), the speed of the transfer amplifier (TA) and ADC has to satisfy the following equation:
      f    S    =            P      ×      P              (                        1                      f            R                          -                  t          EXP                -                  t          GB                    )      where                fS=sampling rate of TA and ADC        P=number of rows=number of columns        fR=frame rate        tEXP=pixel exposure time        tGB=guard band time between pixel exposure and read out.        
For instance, in high performance optical gaming mouse applications, P=38, fR=12,000 frames/s, tEXP=30 us and tGB=10 us, thus the transfer amplifier (TA) and analog-to-digital converter (ADC) have to work in excess of 30 MS/s which translates to high power consumption in these two blocks. Based on the same pixel array, the transfer amplifier (TA) and analog-to-digital converter (ADC) are configured for 50 MS/s operation, examples of the active average standby current (IDDS) are around 650 uA, 1 mA and 1.3 mA respectively for the pixel array 1, transfer amplifier (TA) 2 and analog-to-digital converter (ADC) 3.